1. Field of the Invention
The present invention relates to an observation optical system used for an observation device having an antivibration function, such as a telescope or binocular.
2. Related Background Art
As an observation optical system for an observation device such as a telescope or binocular, an observation optical system having an antivibration function is disclosed in, for example, Japanese Patent Application Laid-Open No. 10-186228.
This observation optical system has an objective optical system having a first lens unit with a positive power (the reciprocal of the focal length) and a second lens unit with a negative power arranged from the object side in the order named, and the second lens unit is driven in a direction perpendicular to the optical axis to effect the antivibration function.
The above objective optical system has a so-called telephoto type arrangement, which is characterized in that the total length of the objective optical system can be shortened.
In the observation optical system, however, an image inverting system such as an image inverting prism must be placed between the objective optical system and the eyepiece optical system, and a driving mechanism for antivibration must be placed inside the objective optical system. Since they require a certain space, the merit of shortening the optical system by making the objective optical system into a telephoto type system is low.
In this case, in driving the second lens unit of the two-unit optical system to effect the antivibration function, an antivibration sensitivity Si is expressed by the following equation using a magnification β of the second lens unit:Si=(1−β)
In the arrangement having positive and negative lens units arranged from the object side in the order named, since β>1, and hence in order to realize |Si|>1, β>2 needs to be set. This arrangement is not so advantageous in terms of sensitivity. An increase in sensitivity can be attained by increasing β of the second lens unit. However, since the power ratio between the positive and negative lens units excessively increases, many lenses are required for aberration correction.
The arrangement disclosed in Japanese Patent Application Laid-Open No. 2000-352664 (corresponding to U.S. Pat. No. 6,249,380B1) has an objective lens system having a first lens unit with a positive power and a second lens unit with a positive power arranged from the object side in the order named. The second lens unit is driven in a direction perpendicular to the optical system to effect an antivibration function.
The total length of this objective optical system is longer than the focal length of the objective optical system. This makes it possible to ensure a space for an image inverting prism and the like. In addition, a space for an antivibration driving mechanism can be easily ensured.
In the arrangement disclosed in Japanese Patent Application Laid-Open No. 2000-352664, however, the magnification β of the second lens unit falls within the range of 0<β<1, and hence the absolute value of the antivibration sensitivity Si in driving the second lens unit becomes smaller than 1 as indicated by the following equation. It is therefore theoretically impossible to increase the antivibration sensitivity.|Si|=|1−β|<1